As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which an electrode assembly, in which a porous separator is interposed between a cathode and an anode, each of which includes an active material coated on an electrode current collector, is impregnated with a lithium salt-containing non-aqueous electrolyte solution.
A general lithium secondary battery assembly process is performed by finally injecting an electrolyte solution into a battery case after alternately stacking a cathode, an anode and a separator and then inserting the cathode, the anode and the separator into the battery case made of a can or a pouch having a certain size and shape. Here, the finally injected electrolyte solution infiltrates a cathode, an anode and a separator by capillary force. However, due to material characteristics such as a cathode, an anode and a separator which are hydrophobic, and an electrolyte solution which is hydrophilic, substantial time and a difficult process are required until an electrode and a separator are wetted with an electrolyte solution.
In addition, devices or equipment are being enlarged and thereby volume, into which an electrolyte solution is infiltrated, is reduced and area, into which an electrolyte solution infiltrates, increases, and, accordingly, there is a high possibility that an electrolyte solution does not enter into a battery and locally exists outside. The amount of an electrolyte solution in a battery manufactured according to such a process battery is partially insufficient, and thereby battery capacity and performance are dramatically reduced.
To improve electrode wetting properties, methods such as injecting an electrolyte solution at high temperature, injecting an electrolyte solution at added or reduced pressure, or the like are used. However, when the methods are used, an electrode assembly and an electrolyte solution may be transformed and thereby problems such as internal short circuit and the like may occur.
Therefore, there is an urgent need for a method of manufacturing a secondary battery having stability at high temperature and improved wetting properties.